Al,
Even if you begin with identical batteries, you're not using your two batteries equally, you're not putting them into storage with an equal charge, they're not aging equally and as a result they have different impedances.
So for maximum longevity use a dual-bank charger. This way each battery is
profiled and conditioned independently.
If two batteries are of equal age, size and chemistry and are permanently in parallel on the same bank then this only requires a single charger bank.

David
Earmark Marine